The terahertz (THz) frequency range, which lies between the microwave and mid-infrared ranges, offers unique opportunities in a variety of application domains which include medical and security imaging, non-destructive testing, submillimetre-astronomy and the detection of gases.
Over the last decade, technological solutions have emerged that are extremely promising to cover the lack of devices in this part of the electromagnetic spectrum. One of the most important and widespread technique is THz time domain spectroscopy called TDS. It is based around optical ultrafast lasers for the generation and detection of THz pulses. The generation of THz pulses is usually performed through ultrafast excitation of photoconductive antennas.
Referring to FIG. 1, a photoconductive antenna has usually a particular substrate (1), usually a semiconductor, with two electrodes (2) on the substrate (1). The mechanism is as follows: irradiating the gap between the electrodes (2) with ultrashort pulse laser light while applying a voltage across the electrodes (2) causes excited photocarriers to induce an instantaneous current flow between the electrodes (2), and the photoconductive antenna emits a terahertz wave with a broad frequency spectrum. It should be noted that THz-TDS systems can use another photoconductive antenna or an electro-optic crystal as a detector for terahertz waves.
In a typical semiconductor antenna, the particular semiconductor can be selected from compound semiconductors such as GaAs, InGaAs, AlGaAs, GaAsP, and InGaAsP. Further, low-temperature-grown GaAs (LT-GaAs) films, grown in the crystalline form are very commonly used (IEEE J Quant. Elect. 28 2464 (1992)) for short carrier lifetime and large resistance. LT-GaAs is grown as a crystal on a semi-insulating GaAs (SI—GaAs) substrate in most cases. This causes various problems while THz waves pass through the SI-GaAs substrate, such as reduced efficiency of use of the power of the THz waves and spectral narrowing.
To overcome the drawback of spectral narrowing, a multilayered antenna has been developed. This is disclosed in US patent application US 2014/0252379, which discloses a photoconductive antenna that generates and detects terahertz waves, and has a substrate without refractive index dispersion, a buffer layer, a first semiconductor layer, a second semiconductor layer, and an electrode in this order. The substrate is made of Si, the buffer layer contains Ge, and the first and second semiconductor layer both contain Ga and As. The element ratio Ga/As of the second semiconductor layer is smaller than the element ratio Ga/As of the first semiconductor layer.
THz-TDS characteristics, including the dynamic range, bandwidth, signal-to-noise ratio and frequency resolution are closely related to the pulse specifications; consequently its performances are mainly linked to the characteristics of the pulse emitter.
More specifically, a THz-TDS has a frequency resolution limited by the total scanning time, which is mainly limited by unwanted interfering echoes (3) of the emitted THz pulse—see FIG. 2. Indeed, the problem of having echoes arises when the original signal is reflected from discontinuities, such as change in refractive index, on the beam path.
If most of the echoes are manageable (for instance by means of samples, windows electro-optic detection crystals with thicker or wedged dimensions), in the case of photoconductive antenna, an echo arises from the reflection of the original pulse in its own substrate. Because of the short distances of a standard size wafer, it is this echo that limits in practice the spectral resolution of the system. With a photoconductive antenna made out of a 500 μm GaAs wafer (n=3.64 refractive index in the THz range), a THz echo arises after only 12 ps, limiting the resolution to a lower bound of typically 90 GHz (3 cm−1).
Although such a resolution is low enough for many applications, several applications require better performances. For example, spectroscopic methods for the sensing and identification of gases have shown great promise, owing to their inherent non-invasive nature, but also because they are highly selective. Compared to mid-IR spectrum, which consists of complex signature of vibrational and rovibrational transitions, the THz fingerprint of many polar molecules consist of simple rotational spectra with unique spectral signature, which may provide more efficient and accurate detection of many gases. To resolve such spectrums, higher resolution is needed, since most of pure rotational spectrum spacing typically ranges from 0.1 cm−1 to 10 cm−1 (e.g. rotational constant B=2 cm−1 for CO molecule).
For higher spectral resolution in THz-TDS systems, several methods have been proposed to deal with echoes issue. THz anti-reflection coatings have been developed but have several drawbacks: the dielectric ones are wavelength dependent (IEEE Microwave and guided wave letters, Col. 10, No. 7, Jul. 2000 “An anti-Reflection coating for silicon optics at terahertz frequencies” A. J. Gatesman, J. Waldman, M. Ji, C. Musante, and S. Yngvesson), while broadband design might be achieved with thin metal coating (Physical Review B 77, 195405; 2008; “Nanostructured gold films as broadband terahertz antireflection coatings”; Andreas Thoman, Andreas Kern, Hanspeter Helm, and Markus Walther)] but are difficult to realize and introduce important losses.
Alternative approaches apply numerical methods, either by deconvolution with reference signal, or echo cancelation with a deconvolution algorithm.
However, these methods require either a careful calibration through a reference in the first case, or an assumption of the dispersion properties of the substrate in the later. As a consequence, due to complexity of the acquired THz signals, it can add artefacts in calculated spectrum.
Finally, the last method consists simply in processing antennas on a thicker substrate so as to displace the echoes to later times, but does not eliminate them. Moreover, emitted power is distributed between echoes and the main pulse, which in a sense constitutes a power loss.
To overcome the above-mentioned limitations, a need exists for a novel antenna design that intrinsically suppresses echoes that usually originate from antenna's substrate reflection, without any numerical post-processing, and which is not affected by losses.